It is known that .alpha.-olefins of 3 or more carbon atoms, such as propene and butene-1, can be polymerized using catalyst combinations of transition metal compounds and metal alkyls, i.e., to so-called Ziegler-Natta catalysts.
It is furthermore known that certain of these catalyst combinations, e.g., .delta.-TiCl.sub.3 with AlEt.sub.2 Cl [M. Ogushi, CEER 8:49 (1976)], are especially effective in achieving a high proportion of monomer units incorporated in an isotactic arrangement. Due to this stereospecific structure, the thus-produced polyolefins are able to crystallize. This has a favorable effect on a number of properties, e.g., hardness, rigidity, strength. Accordingly, such polymers, primarily polypropylene, have developed into synthetic resins which can be used with great versatility and are manufactured industrially in large amounts.
In addition to isotactic polymers, however, all of these catalysts also yield more or less large quantities of atactic components differing from the former, inter alia, by extensive amorphism, solubility in hydrocarbons and tackiness. The latter are ordinarily isolated during the work up step to which the solvents used in the polymerization are subjected. Originally, these atactic components represented an undesirable and unavoidable by-product, whose appearance was to be minimized. However, due to efforts directed toward exploiting this unavoidable by-product, numerous uses have since been discovered. For example, the atactic polymer produced can be utilized in coating textiles, primarily for carpet backing, and paper. In addition, they can be used in the production of noise-damping materials and adhesives, sealing and caulking compounds, road-marking compositions, as well as bituminous materials which are used in road construction and roof coverings.
The development of novel processes for the production of polyolefins, primarily polypropylene (see, e.g., DOS's [German Unexamined Laid-Open Applications] Nos. 2,621,874 and 2,635,163), has provided a greatly improved exploitation of the catalysts, and also enabled a higher stereospecificity. As a result, the amount of thus-produced atactic material was no longer sufficient to cover all the marketing needs.
Consequently, processes have also been developed for preparing extensively atactic polyolefins. In these methods, a lowering of the stereospecificity can be attained by several measures which sometimes are also employed in combination. For example, in place of the mentioned .alpha.-TiCl.sub.3 catalyst component, a titanium compound can be employed which has been prepared in situ by the reduction of TiCl.sub.4 and/or of titanium(IV) alcoholates (German Patent No. 1,795,483). In another option, aluminum trialkyls or fluorinated aluminum alkyls are employed in place of the diethyl aluminum monochloride (German Patent No. 1,931,421 and Belgian Patent No. 733,692) frequently utilized for the production of polyolefins of high stereospecificity. It is also possible to prepare amorphous products using a Ziegler-Natta catalyst containing magnesium chloride (DOS No. 2,153,520 and German Patent No. 2,329,641). Additionally, the titanium component can be replaced by VCl.sub.4 (Japanese Patent No. 4,005,753).
However, all of these prior-art methods exhibit deficiencies. The exploitation of the catalyst, for example, leaves much to be desired; as a result, the content of catalyst residues in the polymer is relatively high unless more or less expensive scrubbing processes are employed. As is known, the consequence of such additional processes is that the stability of the products is impaired.
Often, the content of isotactic components is still so high that the desired degree of amorphism can be attained disadvantageously, only by polymerization at an elevated temperature of above 100.degree. C. (German Patent No. 2,329,641).